1. Field of the Invention
This invention relates to a method for creating a lithologic model of a portion of the subsurface of the earth from seismic data.
2. Discussion of the Prior Art
The goal of a seismic exploration project is to produce and display a model of the structure and texture of subsurface earth layers within an area of interest. An array of seismic sensors are laid out along a line or a grid of observation stations within a desired locality. An acoustic source is stationed in the vicinity of the sensor array at a specified location. The source injects an acoustic signal into the earth. The acoustic signal propagates downwardly into the earth and is reflected from various earth layers back to the surface where the reflected signals are detected by the sensors. The reflected signals detected by each sensor are recorded for later processing. The sensor array is then moved along the line to a new position and the process is repeated. Customarily, the analog signals from the sensors are recorded after being sampled at intervals such as every two milliseconds and are converted to digital numbers whose magnitudes are a function of analog amplitude. When sufficient seismic data have been acquired, the data are transported to a processing center. Seismic data processing is then done using digital computers. After processing, the data are displayed as wiggle traces or colored bands.
The recorded seismic data are corrected for spherical and geometric spreading and instrumental artifacts; spurious noise and multiple reverberations are removed by suitable deconvolution methods. After reduction to zero offset, the data may be displayed as a two-dimentional cross section of the earth in terms of horizontal spacing of the observation stations and vertical two-way travel time beneath each observation station. The data are shown as individual time-scale oscillograms or traces beneath each station. Significant trace excursions from an average value represent reflected wave arrivals.
The polarity and amplitude of the trace excursions are a function of the differences in acoustic impedance between rock layers, i.e., the reflection coefficient. A reflectivity series is a depth-scale plot of the polarity and magnitude of the reflection coefficients. The acoustic impedance of a rock layer is, in turn, the product of the interval acoustic propagation velocity within a particular rock layer and the density of that layer. The time of arrival of a reflected seismic event is a function of the average acoustic velocity from the surface to the rock-layer interface under consideration. The display of seismic data may be a two-dimentional structural profile of subsurface rock layers as above described or it may take the form of a three-dimensional display of the physiographic features of one or more selected rock layers.
More sophisticated displays may be produced to show lateral changes in the texture of the respective rock layers. Parameters of interest are lateral changes in density, velocity and concomitantly, the reflection coefficients. Such lateral changes are considered to be indicative of rock type (shale, sandstone, limestone) and of rock texture such as porosity and fluid content.
If there is a borehole within an area to be seismically surveyed, a depth log of direct measurements of interval velocity, rock density and other parameters may be made. From such a depth-scale log, a synthetic time-scale seismic recording may be computed. The synthetic seismic recording or seismic trace is useful for demonstrating to the geophysicist what a seismic time-scale trace ought to look like in the presence of the geologic conditions near the bore hole.
In the inverse case, from a recorded time-scale seismic trace and, given certain assumed initial conditions, one can measure seismic velocity and reflection wavelet amplitude. Using those observed parameters one can, by a suitable iterative process, derive a synthetic depth-scale velocity/density log. Such a log may be termed a pseudo acoustic impedance log and may be considered to be a one-dimensional local lithologic model of the earth.
Because the seismic data are band-limited as to frequency, the resolution of the synthetic log for thin layers is poor. A lithologic model conventionally derived from band-limited seismic data is imperfect because conventional methods involve only one-dimensional transformation of individual seismic traces, assume that the data are broadband, and make no allowance for noise.
The resolution of a lithologic earth model depends on the frequency content of the seismic data from which it was derived and on the formation velocity. Depth ambiguities between layers of several tens to as much as one-hundred feet or more may be expected because of the band-limitations of the seismic data.
Conventional seismic modeling techniques may be reviewed from "Interpretive Methods of Stratigraphic Modeling" by A. P. Frink et al., present as paper OTC 2836 at the Offshore Technology Conference, May, 1977. Another paper of interest is "Acoustic Impedance Logs Computed from Seismic Traces", M. Becquey et al., Geophysics, V. 44, N. 9, pp. 1485-1501.